In the currently ongoing information technology revolution, display elements, assemblies thereof such as displays, and other optical devices play an important role as interfaces between people and various types of equipment. Such display elements are constantly required to have higher brightnesses and higher resolutions, and must also be as thin and lightweight as possible and consume minimum energy. Phosphors with high brightness and fluorescence quantum yield are indispensable to meeting such requirements.
Inorganic matrices in which primarily rare earth ions or transition metal ions are dispersed have so far been used as phosphors. These phosphors have been researched extensively and are being constantly improved because rare earth ions and transition metal ions change little over time and are degraded only minimally by light irradiation in comparison with organic pigments. However, the transitions of such ions often have a forbidden character, and hence their emission lifetime is about 1 ms (millisecond). Accordingly, irradiating such ions with intense excitation light in an attempt to achieve higher brightness fails to rapidly convert this light to the desired light, and the phenomenon called brightness saturation occurs. This phenomenon is a major obstacle to ensuring higher brightness through the use of rare earth ions or transition metal ions. To overcome this shortcoming, it is necessary to find novel light-emitting materials that lie outside the traditional and conventional ways of thinking.
In recent years, attention has been drawn to the discovery that surface-treated semiconductor superfine particles emit light with high efficiency. Typical examples of semiconductor superfine particles are particles of Group II-VI compounds whose diameters are on the order of several nanometers. These particles display so-called quantum size effects, with smaller particles having an increase band gap. For this reason, such particles can emit various colors depending on the particle size even when all are irradiated with ultraviolet light of the same wavelength, with smaller particles emitting shorter wavelengths of light. For example, cadmium telluride emits yellowish red light when in the form of particles with a diameter of about 5 nm, and green light when in the form of particles with a diameter of about 3 nm. The emission lifetime of these semiconductor superfine particles is about 10 ns (nanosecond). For this reason, excitation light can be converted to the necessary light five orders of magnitude more rapidly than with a phosphor based on rare earth ions or transition metal ions, and the excitation light can be reabsorbed and emitted, making it possible to ensure extremely high brightness. Another advantage is the ability to promptly follow the ON and OFF cycles of excitation light.
It should be noted, however, that such semiconductor superfine particles have a large surface area because of their small particle size, for which reason reducing the number of surface defects by a surface treatment to suppress radiationless deactivation is important for raising the fluorescence quantum yield. Sulfur-containing compounds can suitably be used for such surface treatments. Typical examples include thiols and other organic surfactants, as well as zinc sulfide. Since semiconductor superfine particles whose surfaces are thoroughly coated using such compounds are incredibly bright, it has been shown in the latest research that emission from each individual particle can be separately detected and optically resolved. This can be regarded as an excellent characteristic unattainable with a rare earth or transition metal phosphor. Another significant advantage of semiconductor superfine particles is that various colors can be emitted in accordance with the particle size by irradiating the particles with light whose single wavelength is shorter, that is, has higher energy, than the band gap. In other words, the advantage of such phosphors is that the excitation wavelength can be freely selected and that even when the same material is used emission with the desired wavelength can be obtained by varying the particle size.
Such semiconductor superfine particles are currently produced by a colloidal method. There are two types of particles: those that are produced in aqueous solutions, and those that are produced in nonaqueous solutions.
A typical example of such superfine particles produced in aqueous solutions is cadmium telluride, which has a fluorescence quantum yield of several percent (Gao, et al., Journal of Physical Chemistry, B, vol. 102, p. 8360 (1998)). The quantum yield value is calculated based on a reported method in which the molar absorbance coefficient and the quantum yield are compared with those of a known pigment molecule (Dawson, et al., Journal of Physical Chemistry, vol. 72, p. 3251 (1968)).
Semiconductor superfine particles produced by this method are however stabilized with a surfactant and dispersed in an aqueous solution, and are essentially incapable of producing a monodisperse system in water without a surfactant, making it difficult to raise the dispersion concentration.
A process by which superfine particles are produced using organometallic pyrolysis is known as a method for producing such phosphors in a nonaqueous solution (Bawendi, et al., Journal of Physical Chemistry, B, vol. 101, p. 9463 (1997)). This method is advantageous in that, for example, a quantum yield in excess of 20% can be obtained with superfine particles of cadmium selenide, and although the resulting superfine particles as such are insoluble in water, coating the surface with ionic organic molecules allows the particles to form a monodisperse system in water and to be handled in the same way as the previously described cadmium telluride particles obtained from aqueous solutions. However, this method requires expensive and complicated experimental equipment. Another drawback is that in most cases superfine particles thus produced remain stable in water for only about a few hours, and this instability is a big factor impeding the practical application of particles produced in nonaqueous solvents.
Thus, surface-coated semiconductor superfine particles in solution emit extremely bright light but are unstable. When left unchanged, superfine particles of cadmium telluride obtained by the aqueous solution method usually aggregate and precipitate in about two weeks in air at room temperature. For this reason, the emission performance cannot be maintained for a long time, and the particles lack practicality as an engineering material.
Several attempts have therefore been made to support and stabilize such surface-coated semiconductor superfine particles in a solid matrix. There is, for example, a report concerning a method for fixing such particles in an organic polymer (Bawendi, et al., Advanced Materials, vol. 12, p. 1103 (2000)). However, polymers used as a matrix have inferior light resistance, heat resistance, and other properties in comparison with silicon-containing glass materials, and gradually permit water and oxygen. The resulting drawback is a gradual degradation of the superfine particles thus fixed. In addition, in a mixture of superfine particles as an inorganic material and a polymer as an organic material, the superfine particles are apt to aggregate if the dispersion concentration of the superfine particles is high, and therefore the phosphor tend to have inferior characteristics as a light-emitting material.
To overcome the drawbacks of such polymer matrices, an attempt has been made to disperse superfine particles in a glass matrix by a sol-gel process using a tetraalkoxysilane (Selvan, et al., Advanced Materials, vol. 13, p. 985 (2001)). In this method, however, the superfine particles are insoluble in water, so usable sol-gel processes are limited and only gelled products can be obtained. In addition, the dispersion concentration of the superfine particles in a glass matrix has a low upper limit of only about 0.1 vol % (about 1×10−4 mol/L when the particle diameter is 3 nm), which is insufficient for obtaining a fluorescence intensity greater than that of the currently used rare earth or transition metal phosphors.